Diamond is an extremely hard, electrically insulating allotrope of carbon which has a very high thermal conductivity, nearly five times the thermal conductivity of copper at room temperature. Diamond is also transparent over a wide range of the electromagnetic spectrum. These properties make diamond an ideal material for wear-protective coatings, optical windows, and efficient heat sinks in large-scale integrated circuits.
Diamond films can be grown by various chemical vapor deposition (CVD) techniques, including, among others, low pressure synthesis of polycrystalline and monocrystalline films. The two most widely used techniques are plasma-assisted chemical vapor deposition (PACVD) and the hot-filament process. In PACVD, a microwave or radio-frequency (of the order of MHz) generated discharge is struck in a gaseous mixture of a hydrocarbon (eg. methane, propane, or acetylene, etc.) and hydrogen. Usually, a substrate material, such as silicon, is placed directly interior to a flowing electrical discharge. The carbon-bearing gas mixture is highly dissociated by the action of the microwave discharge. In turn, various molecular and radical species, as well as ions and electrons are formed eventually leading to the observed diamond thin film production. For successful conventional diamond film synthesis, the gas mixture should contain less than about 5 volume % hydrocarbons. The films are typically grown at a rate of several microns per hour, on monocrystalline silicon substrates at temperatures on the order of 800 to 1000 degrees Centigrade. However, there are several complicating and detrimental effects present including poor control over grain sizes in the polycrystalline diamond films, and inability of other substrate materials to withstand such high temperatures, non-uniform heating, non-uniform gas flow patterns, and etching of the substrate or the developing diamond thin film.
The hot filament process uses a similar gas mixture and a tungsten or thoriated-tungsten filament suspended a few millimeters from the substrate surface. The filament is heated resistively and it serves to excite the gas mixture as well as to heat the substrate. Growth rates by this method are comparable to those by PACVD. Diamond films have also been synthesized by using an oxy-acetylene torch (Hanssen et al., Materials Letters, Vol. 7, Number 7,8, page 289-292, Dec. 1988).
Conventional CVD systems which have utilized carbon monoxide (CO) and hydrogen mixtures as the precursor materials for the deposition of thin carbon films have used electrical discharges, i.e., radio frequency (RF), or microwave. CO-H.sub.2 mixtures cannot be used in the hot filament process because the dissociation energy of the CO molecule is large (about 10 eV). Nevertheless, CO-H.sub.2 discharge systems are desirable because they are simpler and lend themselves to thorough theoretical and experimental study more readily than their hydrocarbon-hydrogen counterparts. Furthermore, it is known that the CO partial concentration can be varied over a wide range (eg. 5% to 80%) with little variation in the deposition rate. (See Ito, Idemitsu Petrochemical Corporation, presentation at the Gorham Advanced Materials Institute Conference on Investment, Licensing, and Strategic Partnering Opportunities, Emerging Technologies Applications and Markets for Diamond and DLC Coatings, Oct. 15-17, 1989, Marco Island, Fla.; and Meyer et al., J. Vac. Sci. Technol. A, Vol. 7, pp. 2325, 1989) This is in marked contrast to the hydrocarbon counterparts which cannot exceed 5 volume % partial concentration of hydrocarbon in the gas mixture. (See Deshpandey and Bunshah, J. Vac. Sci. Technol. A 7 (3), May/June 1989)
Mixtures of CO-Ar and CO-He at pressures ranging from 10 to 100 torr have been optically pumped by lasers at room temperature (Urban et al., Chem. Phys., 130, 389 (1989))
It is known to use a CO laser to optically pump very highly pressurized liquid CO. Such a system, however, is too dense to form a film and cannot deposit diamond.
Prior methods of using lasers to assist in CVD of thin diamond-like films have used the laser to heat the substrate or to photo dissociate the carbon-bearing gas. The optical pumping of CO with a CO laser is also known but not for producing diamond films.
Gaseous mixtures of CO/Ar and CO/N.sub.2 /Ar have been optically pumped at room temperature with a CO laser (J. W. Rich et al. "C.sub.2 and CN Formation by Optical Pumping of CO/Ar and CO/N.sub.2 /Ar Mixtures at Room Temperature", Chem. Phys., Vol. 44, No. 1, Nov. 15, 1979). In this same work, it is also shown that the C.sub.2 product formed in the gas phase is enriched in the C.sup.13 isotope by as much as 20%. Similar enrichments (.about.15%) were also reported by the same authors in another experiment in which the isotope containing products were C.sub.2, CO.sub.2, and C.sub.3 O.sub.2. (Bergman and Rich, Proc. Intl. Conf. Lasers, pp. 265-269, 1980) That the isotope species can receive energy either directly from the beam or by collisions with vibrationally excited C.sup.12 O.sup.16 is demonstrated by the experiment of Anex and Ewing (J. Phys Chem., Vol. 90, pp. 1604-1610, 1986). In their experiment, isotopically pure CO (88% C.sup.13 O.sup. 16 and 12% C.sup.13 O.sup.18) in liquid argon was successfully vibrationally excited up to about level v=20 by irradiating the liquid mixture with beam energy from a CO laser operating on its lowest vibrational transitions (i.e., v=1.fwdarw.v=0, v=2.fwdarw.v=1, and higher).